The present invention relates to a polyethylene2,6-naphthalate film for magnetic recording tapes. More particularly, the present invention relates to a polyethylene2,6-naphthalate film which is suitable for a base film of very thin and high-density magnetic recording tapes, having a Young's modulus of not less than 800 kg/mm.sup.2 in the machine direction and a Young's modulus of not less than 600 kg/mm.sup.2 in the transverse direction, being free from the bleeding of oligomers, and being excellent in heat resistance.
Demand for a longer-time recording magnetic tape, and a reduction in size and weight of recording apparatus has recently been increasing, so development of a magnetic recording tape which is thinner than a conventional one and has a sufficient strength has been waited in expectation.
To meet such demand, a polyethylene terephthalate film which has been strengthened in the machine direction has been used as a base film of a magnetic recording tape. However, the molecular structure of polyethylene terephthalate film strengthened in the machine direction makes it difficult to provide a film having a Young's modulus of not less than 800 kg/mm.sup.2 in the machine direction, and since it is difficult to increase the strength in the transverse direction, it is difficult to produce a film of not more than 7 .mu.m thick having a sufficient strength. A high-density magnetic recording tape having a track recording density of not less than 10,000 cycle/cm such as 8 m/m video tape, digital audio tape, high-band video tape and digital video tape is required to have a very small surface roughness as compared to the conventional magnetic recording tape, therefore the film for a high-density magnetic recording tape must be free from the bleeding of oligomers. In a polyethylene terephthalate film, however, the bleeding of oligomers is inevitable during the stretching step, the heat setting step or in the vacuum evaporation (deposition) or cooling step for manufacturing a magnetic tape, which results in a drop out, and an improvement has been demanded.
Especially, when a magnetic recording tape is manufactured by forming a magnetic layer on a base film by deposition, a film which has higher heat resistance than a terephthalate film has been demanded.
On the other hand, since polyethylene-2,6-naphthalate which has a naphthalene ring in a molecular chain has a rigid molecular structure and exhibits a high crystallizability, a film formed therefrom is expected to be excellent in heat resistance, mechanical strength and dimensional stability, and a magnetic recording tape adopting a polyethylene-2,6-naphthalate film as a base film and the process for producing the same have already been proposed (Japanese Patent Publication No. 48-29541 (1973), Japanese Patent Application Laying-Open (KOKAI) No 50-45877 (1975) and Japanese Patent Publication No. 56-19012 (1981)). However, any of these polyethylene-2,6-naphthalate films disclosed in the prior art do not exhibit a high Young's modulus both in the machine direction or in the transverse directon, and it cannot be said the polyethylene-2,6-naphthalate films of the prior art have adequate strength both in the machine direction and in the transverse direction.
The present inventor has made various researches to provide a film having an adequate strength both in the machine direction and in the transverse direction, which is required for a base film of a very thin and high-density magnetic tape, even if the film is as thin as not more than 7 .mu.m. As a result of the research, it has been found that a film having a density of 1.360 g/cm.sup.3 obtained by the biaxial stretching of polyethylene-2,6-naphthalate having an intrinsic viscosity of not less than 0.40 exhibits a Young's modulus of not less than 800 kg/mm.sup.2 in the machine direction and a Young's modulus of not less than 600 kg/mm.sup.2 in the transverse direction, is free from the bleeding of oligomers and excellent in the dimensional stability. The present invention has been accomplished on the basis of this finding.